1. Field of the Invention
The present invention concerns a method of packaging freeze dried vaccines in syringes and a plug for implementing the method.
2. Description of the Prior Art
So-called bypass syringes designed to contain a freeze dried active substance and a solvent are already known. Generally made from glass, these syringes have a generally cylindrical syringe body with a passage leading to the needle in the anterior part and a piston-plug in the posterior part coupled to a piston rod actuated by the user. The cylindrical cavity inside the body is divided into two parts, namely an anterior part containing the freeze dried active substance and a posterior part separated from the anterior part by an intermediate piston-plug and containing the solvent. A bypass is provided by a localized longitudinal rib in the glass. In an initial position the bypass communicates with the anterior part containing the freeze dried active substance and is inaccessible to the liquid because of the presence of the intermediate piston-plug. When the user depresses the end piston-plug the liquid depresses the intermediate piston-plug until it uncovers the bypass whereupon the liquid is expelled into the first part containing the freeze dried active substance. The dissolved active substance is expelled from the syringe through the needle after the two pistons come into contact.
However, these syringes are not generally used for freeze dried vaccines because these vaccines are extremely fragile, very sensitive to the relative humidity and temperature of their surroundings, so that it is not possible to carry out under industrial conditions in situ freeze drying in a freeze drying machine followed by fitting of the intermediate piston-plug and filling in a second, so-called dispensing machine because of the breakdown in the continuity of the environment. As a result freeze dried vaccines such as the particularly fragile Haemophilus vaccine, for example, must be packaged in syringes with no solvent so that the medical or nursing personnel must first draw up the solvent from a bottle.
The present invention proposes to remedy these drawbacks and to enable the packaging of fragile freeze dried vaccines in bypass syringes without compromising the quality of the vaccine.
Another objective of the invention is to implement this process using conventional freeze drying installations and conventional dispensing installations or installations which require only minor and temporary modification.